This invention relates generally to electrical measuring instruments, and more particularly to instruments of the type employed in aircraft and used to measure fuel levels, temperatures, and pressures at various locations on the craft, or to measure other parameters associated therewith.
For a number of years, aircraft instrument panels have been provided with relatively simple, basic d'Arsonval type movements for monitoring the various engine and speed parameters normally associated with flying and maneuvering such craft. Many prior innovations have been devised over the years, for improving the visibility of the pointers and scales employed with such instruments. Advances in lighting have been particularly noteworthy, since the personnel in the cockpit experience wide lighting conditions, from virtual full sunlight, to very low ambient levels, as at night or at dusk. Since there are hundreds of indicators on a typical instrument panel, and since the pilot must monitor or check a large number of readings during the normal course of operation, it is imperative that a particular instrument be relatively easy to see, and to read at a glance, from a number of different positions both in front of and to the side of the instrument face.
While the conventional analog instruments have functioned generally well over the years, they sometimes left something to be desired as far as providing high accuracy readings quickly. In most cases, in order to obtain a high resolution reading, it was necessary to view the pointer head-on (to eliminate parallax), and thereafter do some additional interpolation of the reading in the event that the pointer was disposed between adjacent hatch marks of the calibrating indicia. While it is often not essential that all readings taken by a pilot be pin-pointed to a high degree, those which did warrant an accurate determination required additional time as well as extra mental effort by the pilot.
Until only recently, the use of digital displays for electronic instrumentation was generally confined to laboratory equipment, due to the relatively complex circuitry that was involved in converting analog quantities into digital information without sacrificing conversion accuracy in the process. Digital voltmenters have gained wide acceptance in the past 10 years or so, mainly due to the recently developed capability of packaging an analog-to-digital converter in a single integrated circuit chip. Such digital instruments provide increased resolution over that obtainable with a d'Arsonval movement, but do have several disadvantages. In the event that a malfunction occurs within the instrument, they often do not give an indication that something is wrong. Instead they may merely display a series of digits in the normal manner, such a display conceivably not bearing a true or accurate relationship to whatever is being monitored.
In other cases, the digital display will respond to a transient or other spurious signal, and will provide incorrect readings during the occurence of such a signal. Finally a digital instrument by itself does not provide the operator or viewer with a good feel for the rates of variations in the parameters being monitored, or provide an indication of trends, in the case that a reading is gradually changing. Thus, by itself, a digital instrument leaves something to be desired
Aside from the disadvantages noted above, prior digital voltmeters have tended to be physically large and bulky; accordingly, such devices were not in keeping with the strict requirements as to small physical size and weight, which are characteristic of aircraft instruments today.
Several attempts have been made to combine a digital and an analog instrument into a single instrument casing. Due to the complexity of the analog-digital converters associated with such devices, the resulting system was often relatively large or heavy as noted above, making it unsuitable for use in aircraft, where physically small and light weight components are considered a must. In instruments of the above type, where the digital display was located on the face of the analog meter, the pointer associated with the latter often obscured one or more of the digits, or at least interfered with their direct viewing, tending to cause confusion on the part of the operator and sometimes giving rise to an error in taking the particular reading. Accordingly, the combined analog-digital devices of the prior art have not met with much acceptance or success, as far as their use in the aircraft field is concerned.